1. Technical Field
The present invention relates to methods of avoiding packet collision caused by hidden terminal problems in wireless communication systems.
2. Background Art
Conventionally, the Carrier Sence Multiple Access with Collision Avoidance (CSMA/CA) method has been known as one of random access control techniques used in digital wireless communication systems. This method is adopted in IEEE802.11 standardized for wireless Local Area Network (LAN).
However, in CSMA/CA, there is a situation where two terminals in a communication area provided by a control device (referred to also as a “control point”) cannot receive radio signals from each other due to the distance between the terminals, influence of an obstacle such as a wall, or the like. Such a problem caused by signal exchange failure is called a “hidden terminal problem”.
FIG. 28 is a conceptual diagram for explaining the “hidden terminal problem”. In FIG. 28, each of a transmitting terminal 30A and a transmitting terminal 30B communicates with a receiving terminal 40. However, there is a possibility that the transmitting terminals 30A and 30B fail to detect signals transmitted from each other by their carrier sences, and therefore transmit signals to the receiving terminal 40 at the same time, thereby causing collision between the signals.
One of the techniques of solving this hidden terminal problem has been disclosed in, for example, Non-Patent Literature 1. As shown in FIG. 29, the transmitting terminals 30A and 30B and the receiving terminal 40 exchange a control signal called a Request to Send/Clear to Send (RTS/CTS). It is thereby possible to reduce a possibility of collision between data signals transmitted from the transmitting terminals 30A and 30B.
Prior to data transmission, the transmitting terminal 30A transmits a RTS signal to the receiving terminal 40. In receiving the RTS signal, the receiving terminal 40 transmits a CTS signal to the transmitting terminals 30A and 30B. The transmitting terminal 30B receives the CTS signal and waits for transmission, stopping signal transmission. On the other hand, the transmitting terminal 30A receiving the CTS signal starts its data transmission. The RTS/CTS signal exchange between the transmitting terminal 30A and the receiving terminal 40 causes the transmitting terminal 30B to wait for transmission. It is therefore possible to reduce a probability of collision between signals transmitted from the transmitting terminals 30A and 30B.
In addition, as shown in FIG. 30, another technique for preventing the hidden terminal problem without using RTS/CTS is disclosed in Patent Literature 1. In this technique, if a terminal is connectable to a plurality of control devices in a communication system, the terminal selects a control device having a low probability of hidden terminal problem to be connected.
In FIG. 30, the terminal scans a frequency channel of a control device 1 to measure traffic statistic on a network, thereby obtaining a channel occupancy. Next, the terminal performs the same scanning for a frequency channel of a control device 2.
On the other hand, each of the control devices 1 and 2 scans a using frequency channel to obtain traffic statistics on the network. Then, based on the traffic statistics, a channel use rate of the terminal is calculated to be provided to the terminal.
The terminal thereby selects a control device to be connected, according to the channel occupancy and the channel use rates which are obtained in the above-described manner.
In other words, by the technique disclosed in Patent Literature 1, under the observation that signals transmitted from all of terminals in a communication area arrive at a control device, a target terminal is connected to a control device determined as having least hidden terminal problem in consideration of a difference between a traffic amount scanned by each of the control devices and a traffic amount scanned by the terminal.